Resin optical waveguides of a multi-mode and a single-mode, which are formed on a printed board and thus rigid or formed on a polymer-made film and thus flexible, have been widely used. The principle of the resin optical waveguide is that a core and clad(s) that are a combination of a plurality of resins having different refractive indices are combined and the core is used as an optical waveguide.
On the other hand, silicon optical waveguides obtained by forming an optical waveguide on a silicon chip have been also widely used (see PTL 1).
Any of the resin optical waveguide and the silicon optical waveguide is formed to have an array shape in a form in which a plurality of waveguides is aligned in parallel in one direction, in order to obtain a plurality of waveguide channels.
Attempts to propagate light between the resin optical waveguide and the silicon optical waveguide have been made. However, in order to realize coupling for efficiently propagating light at a micro level, positioning with high accuracy is required.
In the case of a multi-mode optical waveguide, in coupling between optical waveguides or between an optical waveguide and a multi-mode optical fiber, the size of a core cross-section is large, and the sizes of the core cross-sections or the numbers of openings are substantially equal to each other. Thus, it can be realized at an allowable level of loss so long as positioning accuracy of butting cross-sections with each other is assured in the cross-sections being in contact with each other.
In practice, it has been realized by so-called butt coupling.
However, in coupling between a single-mode resin optical waveguide and a silicon optical waveguide, core cross-sections of each are considerably small, and the sizes of the core cross-section or the numbers of openings are largely different from each other. Thus, performing the butt coupling is difficult.
From this viewpoint, an adiabatic coupling, in which light seeping out in an optical axis direction along the array (also referred to as evanescent light below) is captured and caused to communicate over a predetermined distance in the optical axis direction, has attracted attentions as an alternative method (see PTLs 1 and 2).
FIG. 4 is a perspective view illustrating a configuration example of a composite optical waveguide in which a resin optical waveguide and a silicon optical waveguide are adiabatically coupled. FIG. 5 is a side view illustrating the composite optical waveguide 100 in FIG. 4. FIG. 6 is a transverse sectional view at an adiabatic coupling portion of the composite optical waveguide 100 in FIG. 4. FIG. 7 is a partial enlarged view of FIG. 6. FIG. 8 is a partial longitudinal sectional view of the adiabatic coupling portion of the composite optical waveguide 100 in FIG. 4.
In the composite optical waveguide 100 illustrated in the drawings, a resin optical waveguide 200 and a silicon optical waveguide 300 are adiabatically coupled. The other end side of the resin optical waveguide 200 in the composite optical waveguide 100, which is opposite to the adiabatic coupling portion, is accommodated in a connector 400 for coupling with a single-mode optical fiber or the like. The resin optical waveguide 200 is configured of a core 220 and a cladding 210. The silicon optical waveguide 300 is configured of a core 310 and a cladding 310. They are adhered to each other by an adhesive layer 500. In the resin optical waveguide 200 and the silicon optical waveguide 300, light propagates in the cores 220 and 320.
As described above, in the adiabatic coupling, evanescent light is captured and caused to communicate over the predetermined distance in the optical axis direction. Therefore, the core 220 in the resin optical waveguide 200 and the core 320 in the silicon optical waveguide 300 are disposed to face each other at the adiabatic coupling portion, as illustrated in FIG. 6. However, in the resin optical waveguide 200, the cladding is not provided on a side facing the core 320 of the silicon optical waveguide 300, and thus the core 220 is exposed.
FIG. 7 is a partial enlarged view of FIG. 6 and illustrates a one-to-one positional relationship between the core 220 of the resin optical waveguide 200 and the core 320 of the silicon optical waveguide 300 at the adiabatic coupling portion. In the resin optical waveguide 200 and the silicon optical waveguide 300 illustrated in FIG. 7, portions other than the cores 220 and 320 serve as the dads 210 and 310, respectively.
As illustrated in FIG. 7, at the adiabatic coupling portion, the core 220 in the resin optical waveguide 200 and the core 320 in the silicon optical waveguide 300 are disposed in a state of facing each other and bonded to each other by using the adhesive layer 500 of an epoxy resin or the like.
FIG. 8 is a partial longitudinal sectional view of the adiabatic coupling portion of the composite optical waveguide 100 in FIG. 4, and illustrates a form of light propagation of evanescent light at an adiabatic coupling portion 700.